Control circuit and a method for electrically connecting a load to a power source

ABSTRACT

A control circuit ( 1 ) includes two input terminals ( 2, 3 ) for respectively electrically connecting with the active and neutral conductors ( 4, 5 ) of a mains power source ( 6 ). Two output terminals ( 7, 8 ) electrically connect with a load in the form of a two pin domestic electrical appliance ( 9 ). Conductors ( 4, 5 ) terminate at a domestic power outlet socket, and terminals ( 7, 8 ) are incorporated into a two pin plug for insertion into that socket. That is, terminals ( 7, 8 ) include elongate bundled conductors for extending between the outlet socket and the circuit. Appliance ( 9 ) has a conductive component, in this instance a housing ( 10 ) that, under normal operating conditions, is electrically insulated or otherwise electrically isolated from conductors ( 4, 5 ). The housing provides a reference point for circuit ( 1 ), and is electrically connected to that circuit by way of a conductor ( 11 ).

This application claims priority from PCT/AU03/00983 filed Aug. 4, 2003,which claims priority from Australian provisional patent application No.2002950581, the disclosure of which is incorporated herein by way ofcross reference.

FIELD OF THE INVENTION

The present invention relates to a control circuit and a method forelectrically a connecting a load to a power source.

The invention has been developed primarily for domestic mains wiredinstallations and will be described hereinafter with reference to thatapplication. However, the invention is not limited to that particularfield of use and is suitable for all electrical circuits includingindustrial and commercial installations, DC supply systems, automotive,marine and other voltage supply applications, and for use in hazardousenvironments such as mines, petroleum processing plants, petrol stationsand the like.

BACKGROUND

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

The distribution of electrical power across an electricity grip from ageneration site to a consumption site typically occurs by way of a highvoltage AC signal that is conveyed by a plurality of coextensiveconductors that are suspended on distribution structures such as poles,towers and the like. It is also known to use a neutral conductor thatcoextends with the other conductors for providing a common referenceacross the grid. Each, or most of, the structures provide an earth linkfor the neutral conductor by way of an earth stake that is electricallyconnected to the neutral conductor. This is referred to as a multiplecarting neutral system (MEN) and it provides a convenient means forproviding fault protection for the grid and protection of the assetswithin that grid, as well as protection for those persons who come intocontact with those structures.

At a location close to the consumption site, the voltages aretransformed to a lower AC voltage that is typically delivered to thesite over two wires for a single phase system—an active and a neutral—orfour wires for a three phase system—three actives and a neutral. At theconsumption site use is made of a separate earth which is oftenreferenced from an earth stake at that site. The neutral conductor isalso tied to that earth.

A variety of protection devices have been developed for detecting andprotecting against a fault condition associated with a mains wiringsystems for commercial and domestic sites such as the consumption sitereferred to above. These devices generally detect an over-currentcondition, in that the current in the active conductor exceeds apredetermined threshold, or increases at a rate that suggests there is alow resistance current path between the active conductor and one or bothof the neutral or earth conductors.

Notwithstanding these protection devices, there are still manyelectrocutions and electric shocks that occur due to electrical faults,whether those faults arise from the wiring used at the consumption site,the electrical appliance, the conditions of use of the appliance, or acombination of those factor. As it turns out, the most common form ofelectrocution is due to a person creating a conductive path between theactive conductor and the earth.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

According to a first aspect of the invention there is provided a controlcircuit including:

at least two input terminals for electrically connecting with respectiveconductors of a mains power source;

at least two output terminals for electrically connecting with a mainsload;

a sensor that is responsive to a reference signal being in the range ofabout 1 Volt to 50 Volts for providing a sensor signal, the referencesignal being derived from a voltage differential between one or more ofthe conductors and a reference point that should, in use, beelectrically isolated from the conductors; and

a switching device that is responsive to the sensor signal forprogressing between a first mode and a second mode wherein: in the firstmode the input and output terminals are respectively electricallyconnected for allowing the load to receive power from the source via theswitching device; and in the second mode the input and output terminalsare electrically disconnected for preventing the source from supplyingpower to the load via the switching device.

In an embodiment, the load is an electrical appliance such as: a pump;an electric drill; an electric motor; an iron; a hair dryer; a consumerwhite-good such as a refrigerator, a washing machine, a clothes dryer; acomputer, be that a laptop computer or a desktop computer; a computerperipheral device or stand alone device such as a printer, modem,facsimile machine or the like; a piece of or combination of domestichi-fi equipment; a television or associated hardware; a hot waterkettle; or the like. That is, the load is any device that drawselectrical power and which, under normal operating conditions, is ratedfor use with the level of current protection for which the power sourceis designed.

In an embodiment, the control circuit is a protective device forelectrically isolating the load from the power source once a faultcondition has been detected.

According to a second aspect of the invention there is provided acontrol circuit including:

at least two input terminals for electrically connecting with a powersource;

at least two output terminals for electrically connecting with a load;

a sensor that is responsive to a reference signal being within apredetermined range for providing a sensor signal; and

a switching device that is responsive to the sensor signal forprogressing from a first mode to a second mode wherein: in the firstmode the input and output terminals are respectively electricallyconnected for allowing the load to receive power from the source via theswitching device; and in the second mode the input and output terminalsare electrically disconnected for preventing the source from supplyingpower to the load via the switching device.

In an embodiment, if the reference signal moves outside thatpredetermined range, the switching device remains within the secondmode. In other embodiments, however, the switching device automaticallyreturns to the first mode upon the reference signal moving outside thepredetermined range. In still further embodiments, the sensor, when notproviding the sensor signal, provides no signal. However, in otherembodiments, the sensor provides other signals in addition to the sensorsignal.

In an embodiment the reference signal is a voltage, and thepredetermined range is greater than about 1 Volt. In other embodiments,the predetermined range is greater than about 2 Volts, while in furtherembodiments, the predetermined range is greater hand about 3 Volts.

In an embodiment, the reference signal is an instantaneous voltage.However, in other embodiments, the reference signal is an averagevoltage over a predetermined time. In another embodiment, the referencesignal is a DC voltage.

In an embodiment, the power source provides a nominal source voltage,and the predetermined range has a lower limit that is less than thenominal source voltage. In an embodiment, the lower limit is less than50% of the nominal source voltage. In other embodiments, the lower limitis less than 10% of the nominal source voltage. In still furtherembodiments the lower limit is less than 5% of the nominal sourcevoltage. For example, in one of the embodiments, the nominal sourcevoltage is 110 Volts AC, and the lower limit is about 3.5 Volts AC. Thatis, the lower limit is only about 3.2% (3.5/110) of the nominal sourcevoltage. In other of the embodiments, the nominal source voltage is 240Volts AC, and the lower limit is about 3.5 Volts AC. That is, the lowerlimit is only about 1.5% (3.5/240) of the nominal source voltage.

In an embodiment, the power source is a mains power source, and the loadis a mains load. In other embodiments, however, the load is a DC orother load and the control circuit includes a voltage conditioningcircuit for converting the mains voltage to the required DC or othervoltage required by the load.

In an embodiment, the sensor includes a low voltage relay that togglesbetween a first state and a second state in response to the referencevoltage moving into the predetermined range. In this embodiment, therelay, in the second state, results in the provision of the sensorsignal. Preferably, the relay includes a low voltage DC coil, and thereference signal, or a signal derived from the reference signal, isapplied across that coil. In one of the preferred embodiments, thereference signal is a voltage that is relative to a neutral conductor ofa mains power supply. In other embodiments the reference signal is avoltage that is relative to another conductor such as a negative supplyrail of a DC source.

In an embodiment, the switching device includes a mains relay that isresponsive to the sensor signal for toggling between two states thatrespectively give rise to the first mode and the second mode.Preferably, the mains relay includes a coil that is energized by thesensor signal. Even more preferably, in the second mode, the coil isenergized by the source. That is, when progressed to the second mode,the protection circuit—in absence of being reset—maintains the inputterminals in electrical isolation from the output terminals for suchtime as the mains source is active.

In an embodiment, the control circuit is a protective device forelectrically isolating the load from the power source once a faultcondition has been detected.

According to a third aspect of the invention there is provided a controlcircuit including:

at least two input terminals for electrically connecting with a powersource;

at least two output terminals for electrically connecting with a load;

a sensor that is responsive to a reference signal being within apredetermined range for providing a sensor signal; and

a switching device that is responsive to the sensor signal forprogressing from a first mode to a second mode wherein: in the firstmode the input and output terminals are respectively electricallydisconnected for allowing the load to receive power from the source viathe switching device; and in the second mode the input and outputterminals are electrically disconnected for preventing the source fromsupplying power to the load via the switching device.

In an embodiment, the control circuit is a protective device forelectrically connecting the load to the power source once a faultcondition has been detected.

According to a fourth aspect of the invention there is provided acontrol circuit including:

at least two input terminals for electrically connecting with a powersource;

at least two output terminals for electrically connecting with a load;

a sensor that is responsive to a reference signal being within apredetermined range for providing a sensor signal; and

a switching device that is responsive to the sensor signal forprogressing between a first mode and a second mode wherein: in the firstmode the input and output terminals are respectively electricallyconnected for allowing the load to receive power from the source via theswitching device; and in the second mode the input and output terminalsare electrically disconnected for preventing the source from supplyingpower to the load via the switching device.

In an embodiment, the circuit includes a reset device for progressingthe switching device from the second to the first mode. Preferably, thereset device is manually actuated. However, in other embodiments, thereset device is automated. For example, in an embodiment, the resetdevice is responsive to the absence of the sensor signal for progressingthe switching device from the second to the first mode.

In an embodiment, the sensor moves from an inactive state to an activestate in response to the reference signal being within the predeterminedrange. Preferably, the switching device moves between an inactive stateand an active state in response to the sensor signal. More preferably,the switching device, in the active state, is electrically connected tothe power source.

In an embodiment, the switching device is a relay having a coil that, inthe active state, is energized by the power source.

In an embodiment, the control circuit is a protective device forelectrically isolating the load from the power source once a faultcondition has been detected. However, in other embodiments, the controlcircuit is an actuator for electrically connecting the load with thesource upon the detection of a fault condition or other predeterminedcondition.

According to a fifth aspect of the invention there is provided a methodfor electrically connecting a mains load to a mains power source, themethod including:

providing at least two input terminals for electrically connecting withrespective conductors of the mains power source;

providing at least two output terminals for electrically connecting withthe mains load;

being responsive to a reference signal being in the range of about 1Volt to 50 Volts for providing a sensor signal, the reference signalbeing derived from a voltage differential between one or more of theconductors and a reference point that should, in use, be electricallyisolated from the conductors; and

progressing a switching device between a first mode and a second mode inresponse to the sensor signal wherein: in the first mode the input andoutput terminals are respectively electrically connected for allowingthe load to receive power from the source via the switching device; andin the second mode the input and output terminals are electricallydisconnected for preventing the source from supplying power to the loadvia the switching device.

According to a sixth aspect of the invention there is provided a methodfor electrically connecting a load to a power source, the methodincluding:

providing at least two input terminals for electrically connecting withthe power source;

providing at least two output terminal for electrically connecting withthe load;

being responsive to a reference signal being within a predeterminedrange for providing a sensor signal; and

being responsive to the sensor signal for progressing a switching devicefrom a first mode to a second made wherein: in the first mode the inputand output terminals are respectively electrically connected forallowing the load to receive power from the source via the switchingdevice; and in the second mode the input and output terminals areelectrically disconnected for preventing the source from supplying powerto the load via the switching device.

According to a seventh aspect of the invention there is provided amethod for electrically connecting a load to a power source, the methodincluding:

providing at least two input terminals for electrically connecting withthe power source;

providing at least two output terminal for electrically connecting withthe load;

being responsive to a reference signal being within a predeterminedrange for providing a sensor signal; and

being responsive to the sensor signal for progressing a switching devicefrom a first mode to a second mode wherein: in the first mode the inputand output terminals are respectively electrically connected forallowing the load to receive power from the source via the switchingdevice; and in the second node the input and output terminals areelectrically disconnected for preventing the source from supplying powerto the load via the switching device.

According to an eighth aspect of the invention there is provided amethod for electrically connecting a load to a power source, the methodincluding:

providing at least two input terminals for electrically connecting withthe power source;

providing at least two output terminals for electrically connecting withthe load;

being responsive to a reference signal being within a predeterminedrange for providing a sensor signal; and

being responsive to the sensor signal for progressing a switching devicebetween a first mode and a second mode wherein: in the first mode theinput and output terminals are respectively electrically connected forallowing the load to receive power from the source via the switchingdevice; and in the second mode the input and output terminals areelectrically disconnected for preventing the source from supplying powerto the load via the switching device.

According to a ninth aspect of the invention there is provided a wiringsystem for carrying a mains supply from a mains source having at leasttwo mains conductors, the system being installed at a site andincluding:

a transformer located at or near the site and having one or more primarywindings for connecting to the mains conductors and one or moresecondary windings to provides a site voltage that is substantiallyequal to the mains supply;

at least two site conductors that are installed at the site forelectrically connecting with the one or more secondary windings fordistributing the site voltage to predetermined locations about the site;and

a floating conductor that is installed at the site for providing acommon reference voltage with respect to one or more of the siteconductors.

In an embodiment, one of the mains conductors is a mains neutralconductor that is earthed and one of the site conductors is a siteneutral conductor that is not earthed. Preferably, the wiring systemincludes one or more of the control circuits referred to above. Morepreferably, the transformer is an isolation transformer.

According to a tenth aspect of the invention there is provided a wiringsystem for carrying a mains supply from a mains source having at leasttwo mains conductors, the system being installed at a site andincluding:

a transformer located at or near the site and having one or more primarywindings for connecting to the mains conductors and one or moresecondary windings to provides a site voltage that is substantiallyequal to the mains supply;

at least two site conductors that are installed at the site forelectrically connecting with the one or more secondary windings fordistributing the site voltage to predetermined locations about the site;and

a floating conductor that is associated with a load installed at thesite for providing a reference voltage with respect to one or more ofthe site conductors.

According to a eleventh aspect of the invention there is provided amethod of installing a wiring system at a site for carrying a mainssupply from a mains source having at least two mains conductors, themethod including:

locating a transformer at or near the site and having one or moreprimary windings for connecting to the mains conductors and one or moresecondary windings to provides a site voltage that is substantiallyequal to the mains supply;

installing at least two site conductors at the site for electricallyconnecting with the one or more secondary windings for distributing thesite voltage to predetermined locations about the site; and

installing a floating conductor at the site for providing a commonreference voltage with respect to one or more of the site conductors.

According to a twelfth aspect of the invention there is provided amethod of installing a wiring system at a site for carrying a mainssupply from a mains source having at least two mains conductors, themethod including:

locating a transformer at or near the site and having one or moreprimary windings for connecting to the mains conductors and one or moresecondary windings to provides a site voltage that is substantiallyequal to the mains supply;

installing at least two site conductors at the site for electricallyconnecting with the one or more secondary windings for distributing thesite voltage to predetermined locations about the site; and

installing at the site a floating conductor that is associated with aload for providing a reference voltage with respect to one or more ofthe site conductors.

According to a thirteenth aspect of the invention there is provided acontrol circuit including:

at least two input terminals for electrically connecting with a powersource;

at least two output terminals for electrically connecting with a load;

a sensor having a sensor relay that is responsive to a reference signalbeing within a predetermined range for providing a sensor signal; and

a switching device having a switching relay that is responsive to thesensor signal for progressing between a first mode and a second modewherein: in the first mode the input and output terminals arerespectively electrically connected for allowing the load to receivepower from the source via the switching relay; and in the second modethe input and output terminals are electrically disconnected forpreventing the source from supplying power to the load via the switchingrelay.

In an embodiment, the sensor relay is a low voltage DC relay.Preferably, the sensor relay includes a trigger voltage that is lessthan 50 Volts. More preferably, the trigger voltage is less than 35Volts. In one embodiment, the trigger voltage is less than 12 Volts,while in another embodiment it is less than 5 Volts.

In an embodiment, the switching relay is a main relay. That is, theswitching relay is rated for mains voltages and currents.

According to a fourteenth aspect of the invention there is provided amethod for electrically connecting a load to a power source, the methodincluding:

providing at least two input terminals for electrically connecting withthe power source;

providing at least two output terminals for electrically connecting withthe load;

being responsive with a sensor having a sensor relay to a referencesignal being within a predetermined range for providing a sensor signal;and

being responsive to the sensor signal with a switching device having aswitching relay for progressing between a first mode and a second modewherein: in the first mode the input and output terminals arerespectively electrically connected for allowing the load to receivepower from the source via the switching relay; and in the second modethe input and output terminals are electrically disconnected forpreventing the source from supplying power to the load via the switchingrelay.

According to a fifteenth aspect of the invention there is provided acontrol circuit for a mains load having a component that, in normal use,is electrically isolated from a mains power source, the circuitincluding:

at least two input terminals for electrically connecting with the mainspower source;

at least two output terminals for electrically connecting with the load;

a sensor that is responsive to a voltage between the component and oneor more of the input terminals being within a predetermined range forproviding a sensor signal; and

a switching device that is responsive to the sensor signal forprogressing between a first mode and a second mode wherein: in the firstmode the input and output terminals are respectively electricallyconnected for allowing the load to receive power from the source via theswitching device; and in the second mode the input and output terminalsare electrically disconnected for preventing the source from supplyingpower to the load via the switching device.

In an embodiment, the component is a conductive portion of a housing forthe load. However, in other embodiments, the component is a functionalportion of the load. For example, in an embodiment where the load is anelectric drill, the component is a combination of the stator and rotoras this combination is, in normal use, electrically isolated from mainspower source. That is, the drill includes an electric motor withwindings that, under normal conditions, arm the primary component thatis connected with the mains power source. The rotor and stator, however,should be electrically isolated from that source.

According to a sixteenth aspect of the invention there is provided amethod for electrically connecting a mains load to a mains power source,where the load has a component that, in normal use, is electricallyisolated from the power source, the method including:

providing at least two input terminals for electrically connecting withthe mains power source;

providing at least two output terminals for electrically connecting withthe load;

being responsive to a voltage between the component and one or more ofthe input terminals being within a predetermined range for providing asensor signal; and

being responsive to the sensor signal with a switching device forprogressing between a first mode and a second mode wherein: in the firstmode the input and output terminals are respectively electricallyconnected for allowing the load to receive power from the source via theswitching device; and in the second mode the input and output terminalsare electrically disconnected for preventing the source from supplyingpower to the load via the switching device.

According to a seventeenth aspect of the invention there is provided acontrol circuit including:

at least two input terminals for electrically connecting with a powersource;

at least two output terminals for electrically connecting with a load;

a switching relay having a switching coil that is selectively energisedto progresses the relay between two modes wherein: in one of the modesthe input and output terminals are respectively electrically connectedfor allowing the load to receive power from the source via the switchingrelay; and in the other mode the input and output terminals areelectrically disconnected for preventing the source from supplying powerto the load via the switching relay; and

a sensor relay that is responsive to a predetermined condition forenergising the coil of the switching relay.

In an embodiment, the sensor relay has a low voltage coil that isenergised in response to the fault condition. Preferably, the lowvoltage coil is energised by a DC voltage. More preferably, the lowvoltage coil is energised by a DC voltage of about 1 Volt. In otherembodiments, however, the DC voltage is energised by a different DCvoltage.

According to an eighteenth aspect of the invention there is provided amethod of connecting a load to a power source, the method including:

providing at least two input terminals for electrically connecting withthe power source;

providing at least two output terminals for electrically connecting withthe load;

selectively energising a switching coil of a switching relay toprogresses the relay between two modes wherein: in one of the modes theinput and output terminals are respectively electrically connected forallowing the load to receive power from the source via the switchingrelay; and in the other mode the input and output terminals areelectrically disconnected for preventing the source from supplying powerto the load via the switching relay; and

being responsive to a predetermined condition with a sensor relay forenergising the coil of the switching relay.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’,‘include’, ‘including’, and the like are to be construed in an inclusivesense as opposed to an exclusive or exhaustive sense; that is to say, inthe sense of “including, but not limited to”.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly and not by way of limitation, in the accompanying drawings in whichlike references indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone. In the accompanying drawings:

FIG. 1 is a schematic representation of a control circuit according tothe invention and for use with a two pin electrical appliance and asingle phase low current domestic mains supply having a phase to neutralsupply voltage is about 240 Volts;

FIG. 2 is a schematic representation of a control circuit similar tothat of FIG. 1, but for use with a three pin electrical appliance;

FIG. 3 is a schematic representation of the control circuit of FIG. 2;

FIG. 4 is a schematic representation of a control circuit that issimilar to FIG. 3 but applied to a mains supply where the phase toneutral voltage is about 110 Volts;

FIG. 5 is a schematic representation a control circuit according to anembodiment of the invention for use with a single phase high currentdomestic electrical appliance, where the phase to neutral mains supplyvoltage is about 240 Volts;

FIG. 6 is a schematic representation of a site that is wired to receivethe circuit of FIG. 5 as a protection device for all the appliances atthat site;

FIG. 7 is a schematic representation a control circuit according to anembodiment of the invention for use with a three phase high currentdomestic electrical appliance, where the phase-to-phase mains supplyvoltage is about 415 Volts;

FIG. 8 is a schematic representation of an electricity distribution gridthat provides electric power to a remote site that includes a wiringsystem in accordance with a preferred embodiment of the invention,wherein the site also includes the control circuits of FIG. 6;

FIG. 9 is a schematic representation of a control circuit according toanother embodiment of the invention for use with a single phase lowcurrent domestic electrical appliance, where the phase to neutral mainssupply voltage is about 240 Volts;

FIG. 10 is a schematic representation of a control circuit according toa further embodiment of the invention for use with a single phase lowcurrent domestic electrical appliance, where the phase to neutral mainssupply voltage is about 240 Volts;

FIG. 11 is a schematic representation of a control circuit according toan embodiment of the invention for use with a 12 Volt DC load or a 24Volt DC load; and

FIG. 12 is a schematic representation of a control circuit according toan embodiment of the invention for use with a 24 Volt DC load, or a 32Volt DC load, or a 48 Volt DC load.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, there is illustrated an embodiment of the inventionin the form of a control circuit 1. Circuit 1 includes two inputterminals 2 and 3 for respectively electrically connecting with theactive and neutral conductors 4 and 5 of a mains power source 6. Twooutput terminals 7 and 8 electrically connect with a load in the form ofa two pin domestic electrical appliance 9. While not explicitly shown,conductors 4 and 5 terminate at a domestic power outlet socket, andterminals 7 and 8 are incorporated into a two pin plug for insertioninto that socket. That is, terminals 7 and 8 include elongate bundledconductors for extending between the outlet socket it and the circuit.

Appliance 9 has a conductive component, in this instance a housing 10that, under normal operating conditions, is electrically insulated orotherwise electrically isolated from conductors 4 and 5. The housingprovides a reference point for circuit 1, and is electrically connectedto that circuit by way of a conductor 11. It will be appreciated that,in this embodiment, circuit 1 is mounted within housing 10. However, inother embodiments, circuit 1 is mounted otherwise.

Referring to FIG. 2, there is illustrated another embodiment of theinvention for use with a three pin domestic appliance 14, and wherecorresponding features are denoted by corresponding reference numerals.Particularly, a control circuit 15 is similarly disposed within housing10, although conductor 11 is connected to a third conductor 17 that isbundled together with the other two conductors 4 and 5. If appliance 14includes a metal housing or other major conductive element, this iselectrically connected to conductor 17, either by conductor 11 orotherwise.

While in a typical site having a mains wiring system, conductor 17 isconnected to an earth stake or other earth point at the site, in thisembodiment conductor 17 is left floating to provide a referencesimilarly to that provided by housing 10 of the FIG. 1 embodiment.Otherwise, the operation of circuits 1 and 15 is the same.

Reference is now made to FIG. 3 where there is illustrated a moredetailed schematic of circuit 15. It will be appreciates that with theexception of the connection of conductor 11, circuit 1 is identical.

Circuit 15 includes a sensor having a low voltage miniature relay 19with a 5 nominal Volt DC coil is responsive to a reference signalprovided on conductor 11 being within a predetermined range forproviding a sensor sign to a terminal 21 of relay 19. A switchingdevice, including a mains rated relay 22, is responsive to the sensorsignal at terminal 21 for progressing between a first mode—as shown inFIG. 3—and a second mode wherein: in the first mode the input a 2 and 3are respectively electrically connected to output terminals 7 and 8 forallowing appliance 14 to receive power from source 6 via relay 22; andin the second mode terminals 2 and 3 are electrically disconnected fromrespective terminals 7 and 8 for preventing source 6 from supplyingpower to appliance 14 via relay 22.

Relay 19 includes a coil 25, one end of which is connected to conductor11 via an IN4004 diode 26 and a metal film type 8.2 kΩ resistor 27 thathas a tolerance of 1%. The other end is connected to output terminal 8and, via relay 22, neutral conductor 3. Coil 25 is a low voltage coilwith a notional energising voltage of 5 Volts DC. Typically, however,the coil is sufficiently energised once an instantaneous voltage ofabout 3 Volts appears across coil 25. This particular relay is a highlysensitive low voltage relay and has shown, notwithstanding its lowvoltage rating, to be able to withstand the rigours of being exposed toup to the mains voltage. In this embodiment, where the mains voltage isabout 240 Volts AC, it has been found that relay 19 is able to undertakemany tens of thousands of switching operations without resistor 27 inplace, and many hundreds of thousands when the resistor is in place.

It will be appreciated by those skilled in the art that in otherembodiments other low voltage relays are used having nominal energisingvoltages of other than five volts DC.

Relay 19 includes a contact 28 that is a normally open type contactwhich, when coil 25 is not emergised, extends between terminal 21 andanother terminal 29. Under normal steady state conditions, coil 25 isnot energised, as conductor 11 is floating, and terminal 3 iselectrically connected to the neutral conductor of the mains source.That being so, contact 28 remains as shown in FIG. 3.

Relay 19 also includes a terminal 30 that is electrically connected toterminal 7 and, under normal operating conditions, terminal 2 of source6 via relay 22. When coil 25 is energised due to a voltage appearing onconductor 17, contact 28 toggles to the closed position such that itextends between terminals 21 and 30. This has the effect of connectingterminal 21 to terminal 2, wherein the voltage signal at terminal 21under those conditions defining the sensor signal.

Relay 22 includes two terminals 35 and 36 for respectively electricallyconnecting with conductors 2 and 3. Extending from these terminals arerespective contacts 37 and 38 that are shown when normal operatingconditions prevail, in that they are engaged with respective outputterminals 39 and 40 so as to electrically connect terminal 2 withterminal 7, and terminal 3 with terminal 8.

Relay 22 also includes a coil 44, a terminal 45 that is electricallyconnected to one end of coil, and a terminal 45 that is floating. Theother end of coil 44 is electrically connected to terminal 3—and hencethe neutral conductor—via a manual actuated normally closed reset switch48.

Coil 44 is rated for the voltage provided by source 6, which, in thisembodiment, is 220/240 Volts AC. However; other equivalent relays areused in other embodiments.

Under normal operating conditions, coil 44 is not energized, and contact37 extends between terminals 35 and 39, while contact 38 extends betweenterminal 36 and terminal 40. However, once the sensor signal appears atterminal 21, coil 44 is energized and: contact 17 electrically connectsterminals 35 and 45; and contact 38 electrically connects contacts 36and 40. That being so, terminal 7 is electrically disconnected fromterminal 2, and terminal 8 is electrically disconnected from terminal 3.Hence, applicant 14 is electrically disconnected from source 6.Moreover, while such time as terminals 2 and 3 are electricallyconnected to the mains supply voltage from source 6, coil 44 remainsenergized and thereby maintaining relay 22 in the state other than thatillustrated in FIG. 3.

Circuit 15 also includes a 220/240 Volt miniature type fault lamp 51 inparallel with coil 44 for indicating a fault condition. That is, lamp 51will be illuminated to provide a visual indication that appliance 14 hasbeen subject to a potentially unsafe condition. Additionally, circuit 15also includes in parallel with coil 44 a 275 Volts AC polyester type0.22 μF capacitor 52 to short out any high frequency transients that mayarise during a switching of relay 19 and 22.

Circuit 15 includes a miniature type normally open mains test switch 55that at one side is connected to terminal 7, and at the other side thatis electrically connected in series with a IN4004 diode 56 and a metalfilm type 8.2 kΩ current limiting resistor 57 that has a tolerance of1%. Also included within circuit 15 is a 220/240 Volt miniature filamentlamp 58 that extends between terminals 7 and 8 for providing a visualindication that the mains voltage is available to appliance 14.

In other embodiments use is made of different electrical components orcombinations of components that provide the same functionality.

In use and under normal operating conditions, as shown in FIG. 3,terminal 2 and terminal 3 are respectively electrically connected toterminals 7 and a via relay 22. Lamp 58 is also illuminated due to thepotential difference between terminals 7 and 8.

In the event that there is a fault condition, in that there is apotential difference between conductor 11 and the no conductor 5, thiswill cause coil 25 to become energised. Reasons for such a faultcondition are many and varied, and often depend upon the appliance inquestion. Taking as an example, an appliance such a hairdryer thatincludes motor windings and heater windings that are subject to themains voltage, and a conductive metal casing or housing that isinsulated from the mains voltage and which is electrically connected toconductor 11. In a humid environment the risk of a conductive path beingestablished between one or more of the windings or the heating elementsand the housing is increased. Should that path be established—either dueto condensation or by the appliance being inadvertently immersed inwater—there will immediately appear a voltage differential between theneutral conductor (which is electrically connected to one side of coil25) and the housing (which is electrically connected via conductor 11 tothe other side of the coil). If the conductive path is of a relativelylow resistance, and therefore a high percentage of the mains voltageappears across coil 25, it will very quickly toggle from the state shownin FIG. 3 to the alterative state.

While coil 25 has a nominal energisation voltage of about 5 Volts DC, itwill typically toggle at about 3 Volts DC. Accordingly, as soon as aninstantaneous voltage of about 4 Volts appears on conductor 17, relay 19will toggle from one state to the other. Accordingly, it is only if theconductive path is of a relatively high resistance, such that thevoltage on conductor 17 is less than about 4 Volts in this embodiment,that circuit 15 will remain operating normally. The circuit is able tobe made more sensitive through removing resistor 27, although this mayreduce the useful life of relay 19.

Another alternative for reducing the lower limit at which coil 25energises, is to omit diode 26 and simply have either resistor 27 (ifincluded) or coil 25 directly electrically connected to conductor 11.Preferably, this would also be accompanied by the omission of testswitch 55, diode 56, resistor 57 and diode 26.

Another alternative for reducing the lower limit at which coil 25energises, is to make use of a relay having a coil with a lower nominalenergisation voltage. Clearly, such a relay would also have to withstandthe rigours of being subject to the mains voltages.

The inclusion of diode 26 in this embodiment has the effect of half waverectifying any AC voltage on conductor 17. Accordingly, if the mainsvoltage appears on conductor 17 due to a fault condition, it will onlybe once the positive half cycle of the AC signal progresses that coil 25will be energised and relay 25 toggles from one state to the other. Inany event, due to the low threshold for energising coil 25—that is, 5Volts DC when a typical fault condition will place close to the fullmains AC voltage on conductor 17—it toggles very quickly such that thesensor signal is provided at terminal 21 correspondingly quickly. Theappearance of the full mains voltage at terminal 25 ensures that coil 44is quickly energised and contacts 37 and 38 toggled from the state shownin FIG. 3 to the alternate state. Once so toggled, lamp 51 isilluminated and coil 44 maintained in an energised condition while suchtime as the active conductor 4 is connected to terminal 2.

It his been found by the inventors that the above embodiment allows afault condition to persist for less than about 30 msec for a 60 Hz mainssupply frequency. This includes about 6 to 14 msecs for the faultcondition to be detected and contact 28 to move to the other state.There is an additional delay of about 8 to 16 msec as relay 22 togglestates. Statistically, however, the mean delay is typically less thanthis. This is presently understood to be due to:

-   -   1. Relay 19 having such a low threshold for toggling relative to        the likely fault voltage. That is, once about 30 to 40 Volts DC        appears on conductor 17 relay 19 will toggle. More typically, a        fault condition exposes conductor 17 to substantially all the ms        voltage and as such, coil 25 is quickly saturated. In        embodiments where resistor 27 has a lower resistance, or is        omitted, the reaction time of relay 19 will be improved.    -   2. Coil 44 of relay 22 being exposed quickly to the full mains        voltage: initially via terminal 21; and subsequently via        terminal 45. That is, coil 44 is maintained in parallel with        source 6, with minimal series resistance to limit the energising        current.

The above embodiment has been found to operate more quickly than typicalsolid-state protection equipment that is installed in domesticinstallations.

The longevity of relay 19 is also understood to arise from the fact thateven when the maximum coil current does flow, it will only do so for anextremely short period. Accordingly, there is little time for negativethermal effects to take place within relay 19. With current limitingresistor 27 in place, additional protection is offered.

Once relay 22 toggles state in response to the sensor signal, coil 25will de-energise and contact 28 will return to the normally openposition shown in FIG. 3. Moreover, as terminals 7 and 8 are nowisolated from the mains voltage, lamp 58 will be extinguished,indicating to the user of the appliance that power is not being suppliedto the appliance. In addition, lamp 51 will be illuminated to indicate afault.

If, following a fault condition, the user wishes to reset circuit 15,this is achieved by manually depressing switch 48 to progress it into anopen state. This has the effect of allowing coil 44 to discharge throughcapacitor 52 and lamp 51. Once this occurs, contacts 37 and 38 row tothe state shown in FIG. 3. If terminals 2 and 3 are still electricallyconnected with conductors 4 and 5 respectively, and the fault conditionhas persisted, relay 19 will again, and quickly, provide the sensorsignal at terminal 21. That being so: relay 22 will again toggle toisolate terminals 7 and 8 from source 6. However, if the fault conditionis removed, terminals 7 and 8 will be electrically reconnected withsource 6 via relay 22, and lamp 58 will illuminate to indicate normaloperating conditions.

Switch 55 provides the user with the ability to test circuit 15 forcorrect operation. By manually depressing switch 55 and progressing itto a closed state, the mains voltage at terminal 7 is passed to coil 19,simulating fault conditions. When this occurs, lamp 58 is extinguished,and lamp 51 is illuminated.

It will be appreciated that control circuit 1 operates similarly to thatdescribed above for circuit 15. The substantive difference being thatfor circuit 15 the reference voltage is obtained from the conductor 17which coextends about the site with conductors 4 and 5, and is a sharedreference for all three pin appliances in use at that site. Circuit 1,however, makes use of a reference that is typically a conductorassociated with the appliance and that is available to be, or at risk ofbeing, engaged by the user—for example a metal housing or a drillchuck—and which is, consequently, to remain isolated from the mainsvoltage.

A further embodiment of the invention, in the form of a control circuit60, is illustrated in FIG. 4, where corresponding features are denotedby corresponding reference numerals. Circuit 60 includes a similarconfiguration and operation to circuit 1 and 15, although a number ofthe components are different to best accommodate the lower mains voltageof 110/120 Volts AC as opposed to the 220/240 Volts AC mains voltage ofFIG. 3. The components that are different include: relay 22 which has acoil 44 that is rated to 110/120 Volts; resistors 27 and 57 that are 4.7kΩ metal film resistors of 1% tolerance; and lamps 51 and 58 that arerated at 110/120 Volts. Due to the lower maximum voltages to which relay19 will be exposed during a fault condition, resistors 27 and 57 areable to be of a lesser value and still adequately limit the maximumcurrent in coil 25.

It will be appreciated that circuit 60 is applicable to both the FIG. 1and FIG. 2 configurations, and operates similarly to circuits 1 and 15respectively, save for the lower mains voltages.

For electrical appliances, such as motors for pumps, winches, or otherheavier duty equipment, relay 22 will not have sufficient currentcarrying capacity. In these circumstances, and using circuit 15 as anexample, circuit 15 is replaced by a corresponding circuit 61 that isillustrated in FIG. 5 where corresponding features are denoted bycorresponding reference numerals. The components in circuit 61 aresimilar to those in circuit 15, and are also arranged similarly.However, relay 22 has been substituted by relay 62 that includes fourcontact sets, three of which are used in parallel to pass mains voltagesand currents to the appliance or other load. This parallel combinationof contacts increases current rating of circuit 61 over that of circuit15. In other embodiments, however, an increase in current capacity isachieved through use of a higher current rated relay, or through placingtwo relays 22 in parallel. Additionally, higher current capacities areachieved in alternative embodiments by placing two or more like relays62 in parallel.

An alternative use of circuit 61 is shown in FIG. 6. Particularly,circuit 61 is disposed between an isolation transformer 63 and circuit15. It will be appreciated that all the mains current for the site isdrawn through relay 61, and a number of appliances 14 are connected inparallel at the site, some with an associated circuit 15, otherswithout. If a fault condition exists, it will not only ensure that aprotected appliance—that is an appliance with a dedicated circuit 15—isisolated, but also that all appliances are isolated due to circuit 61triggering. That is, the fault condition is common to all controlcircuits due to the shared nature of conductor 17. It will beappreciated that conductor 17 is not earthed but floating, whereassource 6—and the associated transmission lines and generationfacilities—and the primaries of transformer 63 are earthed in accordancewith the standards of a multiple earthing neutral (MEN) wiring system.

While the above embodiments have focus upon single phase electricalloads such as domestic or commercial appliances, the invention is alsoapplicable to three phase loads such as motors, pumps, air conditioningand heating units, and the like. Reference is made to FIG. 7 where thereis illustrated a control circuit 71 and where corresponding feature aredenoted by corresponding reference numerals. Circuit 71 is similar tocircuit 15, although some of the components have been varied due to thehigher voltages involved. That said: the configuration of two relays—onelow voltage relay connected to the neutral conductor, and a mains relaythrough which the load current flows—is retained. As there are threeactive phases, relay 22 is substituted by a relay 72 that includes fourcontacts, of which accommodate the respective phases.

As coil 25 of relay 19 is, at worst, exposed to a phase-to-neutralvoltage, resistors 27 and 57 remain unchanged from the FIG. 3embodiment. The phase-to-neutral voltage in the context of a mainssupply is reference to the voltage between an active conductor and theneutral conductor. For a single-phase supply there is only one “phase”or active conductor, while in a multi-phase supply there are usuallythree “phase” or active conductors. Other component changes include:relay 72 being rated at 415/440 Volts AC; diodes 29 and 56 beingupgraded to IN4084 type with a 600 Volts AC rating; capacitor 52 beingupgraded to a 1000 Volts AC rating; and filament lamps 51 and 58 beingupgraded to 415/440 Volts AC ratings. Otherwise, the operation ofcircuit 71 is very similar to that of circuit 15.

Reference is now made to FIG. 8 where there is illustrated schematicallya further embodiment of the invention and where corresponding featuresare denoted by corresponding reference numerals. Particularly, FIG. 8illustrates power source 6, in the form of an electric power generationsite, and a remote power consumption site 81, as represented by the areabounded by the correspondingly numbered broken line. Source 6 and site81 are electrically interconnected by a power distribution system 82that includes a plurality of spaced apart distribution structures 83(only one shown for clarity) such as towers, poles and others thatsuspend or otherwise support the plurality of conductors that carry theelectrical power between the generations site and the consumption site.Each or most of the distribution structures include an earth stake orother earthed reference that is linked to the neutral conductor that issupported by that structure. This provides an MEN system.

However, at site 81, the electrical power provided by source 6 is fed totransformer 63, where it appear on the secondary windings as an isolatedsource. In prior art wiring systems, conductor 17 at a consumption sitewould be earthed. However, in the present embodiment, site 83 includes aconductor 17 that is floating, and which is available to be electricallyconnected with one or more elements of one or more appliances to besupplied power at site 83. This common floating reference across aplurality appliances at that site allows all control circuits of thepreferred embodiments used at that site to be responsive to a fault andthereby provide an increased level of safety. As discussed above, thecontrol circuits of the preferred embodiment are able to be adapted forintegration with a specific electrical appliance or installed in anoverall protective role in a switchboard or the like.

It will be appreciated that there are a plurality of appliances 14 atsite 81, each of which is connected in parallel with the terminals 2 and3. Some of these appliances, such as the one illustrated includeindividual control circuits 15 or the like, while other appliances donot. In any event, in the presence of a fault condition, those latterappliances will still be isolated due to the operation of circuit 63.

It will be appreciated that, as with the prior art, if a fault conditionarises at a two pin appliance used at site 83 that does not have aseparate control circuit in accordance with the invention, the faultcondition will remain until a conductive return path to the neutral isestablished.

A further embodiment of the inventions is illustrated in FIG. 9, wherecorresponding features are denoted by corresponding reference numerals.Particularly, a control circuit 90 is intended for disposing betweensource 6 and a load in the form of appliance 14 in substation forcircuit 1 of FIG. 1. In other embodiments, circuit 90 is configured forinstallation in a switchboard of a power consumption site 83 orotherwise. However, in this embodiment, circuit 90 is designed for alower current application and to a specific species of electronicappliance that is sensitive to over or under voltage conditions.Examples of such appliances include specific computer devices andperipherals, various communications devices, precision measuringequipment and other electronic devices such as high quality video andsound reproduction equipment. Typically, devices that process signalsdigitally have an accuracy of processing that is sensitive to variationsin power supply voltages. However, there are also many analogue devicesthat are similarly sensitive.

Circuit 90 includes a 275 Volts AC 20 Amp varistor 91 that has a 20 mmdiameter and which is connected in parallel with terminals 2 and 3 fordetecting a severe over-voltage condition—in the order of 275 Volts—atthose terminals. In the presence of such an over-voltage condition,varistor 91 reverts to a low resistance state resulting in a largecurrent being passed through the terminals and the triggering of astandard current protection device that is disposed further back alongthe power supply chain. Accordingly, an over-voltage will not be passedthrough to the appliance or other load that is connected in parallelwith terminals 7 and 8.

Circuit 90 also includes a mains relay 93 having a coil 94 that, undernormal operating conditions, is energised by the mains voltage tomaintain three contacts 95, 96 and 97 in the positions shown in theFigure.

A 240 Volt primary center tapped 9/0/9 Volts AC 150 mAmp transformer 98is placed in parallel with terminals 2 and 3 for providing an AC voltageof between 0 and 6 Volts via:

-   -   A bridge rectifier, in the form of a WO4 mini bridge rectifier        99 that is rated at 400 Volts (max).    -   An RB electrolytic capacitor 100 that is rated at 100 μF at 25        Volts DC.    -   A IN4004 diode 101.    -   A 6 Ω 1 Watt resistor 102 that has a tolerance of 10%.

This voltage is regulated by a voltage regulator 103 that, in thisembodiment, is a 7805 regulator of the positive type. Regulator 103 hasa set point that is able to be adjusted by correspondingly adjusting a10 kΩ linear potentiometer 104 that is rated at 0.5 Watts. Thisadjustment allows circuit 90 to be more easily applied to a variety ofappliances, or to different mains voltages.

A IN4004 diode 110 and a RB electrolytic 220 μF.25 Volts DC capacitor111 filter the voltage provided by regulator 103 and apply the resultantfiltered voltage provided by regulator 103 and apply the resultantfiltered voltage to one end of a coil 115 of a miniature type relay 116.The other end of coil 115 is connected in series with resistor 102.Relay 116 is like relay 19.

For predetermined values of voltages at between terminal 2 and 3, coil115 will remain de-energised. However, once the input voltage movesoutside that range, the voltage provided by regulator 103 will rise suchthat coil 115 is energised sufficiently to toggle a contact 118 from thestate shown to the alterative state. Once that occurs, coil 94 of relay93 will be floating and hence that coil will be de-energised, andcontacts 95, 96 and 97 will toggle to their respective alternativestates. This, in turn, will isolate terminals 7 and 8 from terminals 2and 3, while also ensuring that lamp 51 is illuminated, while lamp 58 isextinguished.

Circuit 90 also includes a normally closed reset button 117 for allowinga user to manually reset the circuit after a fault condition has beenremedied.

A further embodiment of the invention, in the form of a control circuit120 is provided in FIG. 10, and where corresponding features are denotedby corresponding reference numerals. Circuit 120 includes terminals 2and 3 that are electrically connected to a 240 Volts mains power sourceand terminals 7 and 8 that are connected to a load in the form of analarm siren and warning lights (not shown). In other embodiments, theload is a different device or appliance. While the embodiments referredto above under normal operating conditions ensure the mains voltage issupplied to the load and interrupt that supply only if predeterminedconditions exist, this embodiment operates in a different sense. Moreparticularly, under normal operating conditions the mains supply isisolated from the load and it is only under predetermined conditionsthat that mains supply is connected with the load.

The circuit also includes a sensor in the form of a conductor 121 thatis electrically connected with an external component that, underpredetermined conditions, provides a voltage signal. In this embodiment,conductor 121 is electrically connected to a component in the form of acommon 9 Volt domestic smoke detector (not shown). Under smoke freeconditions, the detector does not apply a voltage to conductor 121.However, when the detector detects a smoke condition it provides a 9Volts DC signal to conductor 121 which, in turn, is feed by seriescurrent limiting resistor 122 to an LED side of a triac opto-coupler 123such that the coupler is progressed to a conductive state. Resistor 122is a 56 Ω 0.5 Watt metal film type resistor, while coupler 123 a typeMOC 3020/3021 that is contained in a 6 pin package.

Coupler 123, in the conductive state, allows a current flow sufficientto trigger a triac 124. Two biasing resistors 125 and 126 are chosen toset the sensitivity of triac 124, while a capacitor 127 is used to shortany undesirable switching transients. In this embodiment, traic 124 is atype BT 137F (in a TO-220 case) that is rated at 600 Volts AC and 8Amps. Resistors 125 and 126 are each 470 Ω metal film resistors that arerated at 0.5 Watts. Capacitor 127 is a polyester type capacitor that israted at 0.01 μF at 600 Volts AC. In other embodiments alternativecomponents and values are used.

A miniature 5 Volts DC relay 130 includes coil 131 that, upon triac 124being triggered, is quickly and easily energised as it is subjected tosubstantially the full mains voltage. This toggles a contact 132 fromthe state shown in FIG. 10 to the alternative state (not shown). Relay130 is similar to relay 19, but in other embodiments alternative relaysare used.

A mains rated relay 135 includes a coil 136 that under normal conditionsis not energised. However, once contact 132 of relay 130 toggles to thealternative state, coil 136 is subject to the full mains voltage andquickly toggles its two contacts 137 and 138 from the normal stateshown, to the alternative state (not shown). Once this occurs, the mainsvoltage appearing at terminals 2 and 3 is transferred to terminals 7 and8. As such, the alarm siren and warning lights are provided with mainspower and activated to alert those in the vicinity of the smokecondition. In other embodiments, however, the alarm siren and warninglights are at a location other than near the smoke condition.

Once relay 135 switches or toggles between states, coil 131 of relay 130de-energises and contact 132 returns to the state illustrated in FIG.10.

Circuit 120 also includes an output signal conductor 139 that isconnected to external circuitry (not shown). Accordingly, with conductor121 being held at or about 9 Volts, and relay 135 toggled from theresting state, the external circuitry is alerted to the smoke conditionand is able to amplify the condition. For example, the externalcircuitry is, in one embodiment, an actuator for exit signs at the samesite where circuit 120 is installed.

Circuit 120 is advantageous because there is very little current loadplaced upon the smoke detector. Moreover, the battery need supply thatcurrent for only a short period, that is, until coil 131 is energised.As with the other embodiments, the fact that coil 131; is a nominal 5Volts DC coil; and is being briefly exposed to a substantive voltage,the delay prior to the energising of that coil is very short.Accordingly, the lifetime of the 9 Volt DC battery in the detector willnot be unnecessarily compromised.

Circuit 120 also includes a normally closed manual reset button 141 forallowing a user to de-energise coil 136 and hence once again isolateterminals 7 and 8 from terminals 2 and 3. Once that occurs, and assumingthe smoke condition is not still present, the alarm siren and warninglights will be deactivated.

This circuit 120, or an equivalent circuit, is able to be integratedwith an alarm siren, warning lights, or other electrical device orappliance that is desired to be powered upon predetermined conditionsbeing satisfied. In another embodiment, circuit 120 is included withinthe housing of a smoke detector.

Also included in circuit 120 is a normally open manual test button 142that allows manual testing of the circuit.

A further embodiment of the invention, in the form of a control circuit150, is illustrated in FIG. 11, where corresponding features are denotedby corresponding reference numerals. Circuit 150 includes inputterminals 2 and 3 that for electrically connecting with a 12 Volt DCsupply (not shown), and output terminals 7 and 8 for providing thatsupply voltage to a 12 Volt load (not shown). This circuit is primaryfor automotive applications, but is also suitable for domestic andcommercial use. In this embodiment the load is an electric motor, andcircuit 150 is for preventing that motor from starting in the event thatthe starting current is too great. Such a condition occurs, for example,if the motor is heavily loaded.

Circuit 150 includes a two relay combination of a 12 Volt DC relay 151and a 5 Volt DC relay 152 that are configured similarly to relay 22 andrelay 19 respectively of FIG. 3. However, due to the different operatingvoltages, the components are different. For instance: capacitor 52 is a0.22 μF device that is rated at 30 Volts DC; filament lamps 51 and 58are rated at 12 Volts DC; and resistors 27 and 57 are 22 Ω metal filmresistors.

Also included within circuit 150 is a normally open manual switch 153 inseries with diode 26, and a 0.5 Ω 30 Amp resistor 154 disposed betweenterminals 3 and 8, and in parallel with coil 155 of relay 152. Onceswitch 153 is depressed to commence the starting operation, should theload current of the motor—which also must flow through resistor154—generate a sufficient voltage across coil 155, that coil will beenergised and subsequently toggle relay 151 from the state shown to thealternative state. This, in turn, will disconnect the motor from thepower source and allow the user to investigate and/or remove the causesthat give rise to the fault condition.

Circuit 150 is applicable to other voltage supplies and loads other thanmotors. The application of this particular circuit to a 24 Volt DCsupply requires an up-rating only of the coil in relay 115 and lumps 51and 58. It will be appreciated that it may also be necessary to adjustthe values of resistors 27 and 154 to ensure coil 155 of relay 152 isenergised at the correct threshold.

A still further embodiment of the invention, in the form of a controlcircuit 160, is illustrated in FIG. 12, where corresponding features aredenoted by corresponding reference numerals. Circuit 160 is similar tocircuit 150, although it includes input terminals 2 and 3 forelectrically connecting with a 24 Volt AC supply (not shown), and outputterminals 7 and 8 for providing that supply voltage to a 24 Volt load(not shown). In other embodiments, circuit 160 is applied to othersupply voltages, including particularly 32 Volts AC and 42 Volts AC.However, many other supply voltages are also accommodated with theappropriate selection of components that are rated for those voltages.This circuit is primary for mine applications, but is also suitable fordomestic and commercial use, as required. In this embodiment the load isan electric motor (not shown), and circuit 160 is for preventing thatmotor from operating in the event of a fault condition comprising ashort between the active and neutral conductors of the power source.

In marine applications, a primary concern revolves around water, and theability for water to interfere with electric and electronic equipmentsuch as the motor in this embodiment. Accordingly, use is made of acontact pad 161 that is comprised of two opposed stainless steel platesthat are mounted on Bakelite material or other substrate. This pad ismounted at the base of the motor. In other embodiments, however, aplurality of such pads connected in parallel and mounted to the motor atspaced apart locations. As the substrate becomes moist it progressivelybecomes more conductive. Eventually, as the moistness increases, pad 161will act as a short circuit, and, duo to the voltage drop acrossresistor 154, relay 152 will toggle states and the motor will beisolated from terminals 2 and 3.

The control circuits of the preferred embodiments are able to beconstructed cost effectively from “off-the-shelf” components that aremade by leading international electric and electronic manufacturers.This makes it easy to justify the inclusion of a control circuit withina wide variety of appliances. Moreover, the assembled components arerelatively compact—with typical total dimensions of about 30 mm×20 mm×10mm—and able to be relatively easily fitted within the existing housingsof a variety of typical appliances such as white goods or other domesticappliances. Alternatively, if some redesign is required, it need not besubstantive.

The functionality and flexibility of the circuits of the preferredembodiments allow the invention to be applied not only directly intoindividual appliances, but also as a protection device for a pluralityof appliances at a given site. In the event of a fault condition, thislevel of protection will also reduce the risk of electrocution orelectric shocks. This applies even if the fault condition is caused by athree pin appliance at the site that does not itself have a dedicatedcontrol circuit according to the invention.

The embodiments used with AC mains supplies will, upon a fault conditionbeing present, trigger within about 30 msec at a mains frequency of 60Hz. Accordingly, the risk of electrocution or electric shock is at leastas good as, and typically better, than commercially available prior artprotection devices. Additionally, these embodiments monitor the voltageacross the neutral conductor and a floating conductor. The floatingconductor is electrically connected to those surfaces or elements that auser or other person is likely to contact and, as such, should thatsurface or element become electrified, the control circuit concernedwill trigger. This is not over-current protection, but rather theprevention of a fault current in the first place. Prior art protectiondevices have typically protected against over-current conditions.However, that will still allow a large fault current to flow, at leastinitially, when a fault condition exists. The time that the faultcondition is allowed to persist is critical when protecting againstelectrocution. It is in this area of minimising the quantum and durationof the other currents where the embodiments of the invention areparticularly advantageous.

Some of the embodiments of the invention are configured such that therelays coils and as many of the other components as possible are at arest condition—that is, a low power consumption condition—a duringnormal operating conditions. It is only once a fault condition occursthat the relays are activated and consume any substantive power.Moreover, the low voltage DC relay is only in a high power consumptionmode for a very short time, for as soon as the mains relay switches, theDC relay is once again returned to a low power consumption state.

The embodiments of the invention lend themselves to being encapsulatedfor both electrical and physical isolation from surrounding electricalcomponents. This allows immersion of the control circuits in water orother fluids, in use in other hazardous environment without corruptingtheir correct operation. This arises from the fact that the embodimentsof the invention, under normal operating conditions, consume onlyminiscule amounts of energy and hence radiate correspondingly minisculeamounts of heat. This will ensure that the encapsulating material willnot be prematurely degraded, and that no thermal build up willdeteriorate the elements of the control circuit itself.

From the teaching herein it will be appreciated by those skilled in theart that it is possible to select components and combinations ofcomponents for the control circuits of the invention to allow thoserespective circuits to be applied to voltages and loads not expresslymentioned herein.

Although the invention has been described with reference to specificexamples it will be appreciated by those skilled in the art that it maybe embodied in many other forms.

1. A control circuit including: at least two input terminals forelectrically connecting with at least two conductors of a power source;at least two output terminals for electrically connecting with a load; asensor having a sensor relay that is energized in response to areference signal being within a predetermined range, the referencesignal being derived from a voltage differential between one or more ofthe conductors and a floating reference point that is electricallyisolated from the conductors when in use, wherein the sensor provides asensor signal in response to the sensor relay being energized; and aswitching device having a switching relay that is responsive to thesensor signal for progressing between a first mode and a second modewherein: in the first mode the input and output terminals arerespectively electrically connected for allowing the load to receivepower from the source via the switching relay; and in the second mode,the input and output terminals are electrically disconnected forpreventing the source from supplying power to the load via the switchingrelay.
 2. A circuit according to claim 1 wherein the sensor relay is alow voltage DC relay.
 3. A circuit according to claim 2 wherein theswitching relay is a mains voltage relay.
 4. A circuit according toclaim 2 wherein the switching relay is a DC voltage relay.
 5. A circuitaccording to claim 1 wherein the sensor signal is: an AC signal; orderived from an AC signal.
 6. A control circuit including: at least twoinput terminals for electrically connecting with a power source; atleast two output terminals for electrically connecting with a load; aswitching relay having a switching coil that is selectively de-energisedand energised to respectively progress the switching relay to a firstmode and a second mode wherein: in the first mode the input and outputterminals are respectively electrically connected for allowing the loadto receive power from the source via the switching relay; and in thesecond mode, the input and output terminals are electricallydisconnected for preventing the source from supplying power to the loadvia the switching relay; and a sensor relay having only one sensor coil,the sensor coil being progressed to an energized state in response to afault condition for energizing the switching coil.
 7. A circuitaccording to claim 6 wherein the sensor coil is a low voltage coil thatis energised in response to the fault condition.
 8. A circuit accordingto claim 7 wherein the low voltage coil is energised by a DC voltage. 9.A circuit according to claim 8 wherein the low voltage coil is energisedby a DC voltage of greater than one Volt.
 10. A circuit according toclaim 6 wherein the switching relay has only one switching coil.
 11. Acircuit according to claims 6 wherein by progressing to the energizedstate, the switching relay de-energizes the sensor coil.